A thing that crawls or moves at a slow pace, especially an insect.

A crawler is a program that visits Web sites and reads their pages and other information in order to create entries for a search engine index. The major search engines on the Web all have such a program, which is also known as a "spider" or a "bot." Crawlers are typically programmed to visit sites that have been submitted by their owners as new or updated. Entire sites or specific pages can be selectively visited and indexed.

Crawler-based search engines have three major elements. First is the spider, also called the crawler. The spider visits a web page, reads it, and then follows links to other pages within the site. This is what it means when someone refers to a site being “spidered” or “crawled".

Web crawlers/Web spider are mainly used -

1)to create a copy of all your visited pages for later processing by a search engine that will index the downloaded pages to provide fast searches.
2)for automating maintenance tasks on a Web site, such as checking links or validating HTML code.
3)to gather specific types of information from Web pages, such as harvesting e-mail addresses (usually for sending spam)

This is an automated script browse the world wide web in methodical manner up to date data to the particular search engine.

Great explanation.

Crawler is a program that visits the websites and read their pages and other information in order to create entries for a search engine Index

A web crawler is a program which crawls and index the website.

Crawling is the process to follow a path. crawling means following your links and crawling your Website by the search engine and a process by search engine index is where the data that search engines collect is stored & spreading in many websites is called indexing.

A Web crawler is an Internet bot which systematically browses the World Wide Web, typically for the purpose of Web indexing

crawler is google bot made to check all website content.

Crawler is the search engine bot used by the search engine to crawl our website to get the traffic analysis and page ranking results analysis.

A search engine crawler is a program or automated script that browses the World Wide Web in a methodical manner in order to provide up to date data to the particular search engine. While search engine crawlers go by many different names, such as web spiders and automatic indexers, the job of the search engine crawler is still the same.

A net crawler is a web larva that helps in Web categorization. They crawl one page at a time through an internet site till all pages are indexed.